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Full text of "Text Book Of Mechanical Engineering"

Appendix III.

931

y   «

=*     1-405285

By using this quantity,    J = yy8

P. 625. Nominal H.P. of Engines and Boilers.—The

proper method of stating power is undoubtedly by actual per-
formance, and thus we obtain the LH.P. and B.H.P. respectively.
Nevertheless, although the older * nominal' H.P. has fallen into bad
odour through its gradually receding so far from the actual H.P.,
it is extremely convenient, for rating purposes, to have some rule
for estimating the power of an engine, based on general dimen-
sions, especially where an actual test is inconvenient. Neither
can there be any objection to this, so long as the rule
approximates to truth, and is never preferred to the latter.

Engine H.P. — The North East Coast Institution made a
careful examination of marine engines and boilers in 1888, from
which they devised a rule for Normal Indicated H.P. as they
termed it, thus

NY    TT   T"»                       V"^      \/^',J^^/   AV
. I. H. P.   =...............-.....'----

TOO

where   D2 = added squares of every cylinder diameter, in inches.
S    = piston speed in feet per m.
H   =s heating surface of boiler, in square feet.
P   = working pressure in boiler, in Ibs. per square inch.

Of course, this rule was based on the practice of 1888, and
might require alteration from time to time, mainly on account
of altered practice regarding expansion.

Boiler H.P.—By the horse power of a boiler we mean that
power which would be developed if the boiler were to supply
steam to an average engine. To make a simple rule, certain
average constants must therefore be assumed, which are

C  = coal burnt per sq. ft. of grate surface per hour, say 20 Ibs.
E « water evaporation per Ib. of coal burnt, say 8 Ibs.
S   = weight of steam used per LH.P. per hour by the engine
connected with the boiler, say 20 Ibs.

Suppose, then, we require to know the LH.P. obtainable from